NaY molecular sieves are generally synthesized from an alkaline silicon-aluminum gel system. The raw materials are generally water glass, aluminum sulfate, sodium meta-aluminate, and a directing agent. The molecular sieves synthesized with these materials are characterized by a high content of molecular sieves and a high silicon/aluminum ratio. Different modification methods enable such molecular sieves to have diversified reaction characteristics. NaY molecular sieves are the major source of the active components in the field of catalytic cracking.
A new NaY molecular sieve synthesis method, referred to as the kaolin in-situ crystallization method, was first developed in the 1970s. The in-situ crystallization zeolite technologies disclosed by U.S. Pat. Nos. 3,506,594, 3,503,900, and 3,647,718 take kaolin as a raw material for simultaneous preparation of active component and matrix, wherein FCC catalysts obtained by in-situ crystallization are called kaolin-based catalysts. The main preparation process thereof involves spraying kaolin into microspheres fit for a Fluidized Catalytic Cracking (FCC) unit. After the microspheres undergo calcinations, a portion of the microspheres are subsequently transformed into NaY molecular sieves in alkali system, and then, after modification, are prepared into FCC catalyst.
Compared to FCC catalyst prepared by semi-synthesis technology with gel-synthesized Y-type molecular sieves as active component, kaolin-based FCC catalysts have an advantage of stronger resistance to heavy metal, higher activity, excellent hydrothermal stability, and structural stability.
In the early phase, meta-kaolin microspheres are generally used for synthesizing NaY molecular sieves. United Kingdom Document UK1271450 discloses kaolin calcined into meta-kaolin at temperature of below 704° C. and then reacted with sodium silicate to synthesize Y zeolite. U.S. Pat. No. 3,377,006 discloses the synthesis of Y zeolite with specially fine meta kaolin powder.
U.S. Pat. Nos. 3,367,886, 3,367,887, 3,506,594, 3,647,718, 3,657,154, 3,663,165, and 3,932,268 disclose that, in in-situ crystallized products prepared with kaolin calcined at high temperatures above 900° C. (hereafter referred to as “kaolin calcined”), Y zeolite has a high ratio of silicon to aluminum, but crystallinity is less than 30%, and generally in the range of between 20% and 30%.
U.S. Pat. No. 4,493,902 discloses a process for the production of a crystallized product with high content of zeolite using microspheres with the coexistence of meta kaolin, kaolin calcined and crystal seeds in a same microsphere. The crystallinity of the crystallized product is greater than 40%. However, the disclosed process places substantial requirements on the raw materials used for spray shaping in that such raw materials should be super fine kaolin calcined Satone No. 2 and super fine raw kaolin clay ASP-600. Such super fine materials are very expensive and are difficult to obtain from commercial sources.
European Document EP369629 discloses increasing microsphere pore volume by increasing fine powder kaolin calcined, so as to increase the zeolite content in the microsphere. The disclosed process can increase the crystallinity of products to 70%. However, such super fine powder of kaolin calcined is very expensive. In addition, substantial amounts of kaolin calcined with low adhesive property are present in the sprayed microspheres, resulting in microspheres of poor attrition resistance.
Peoples Republic of China Document CN1232862 discloses a method for the preparation of high-content in-situ crystallized molecular sieve. Pore structure of the crystallized product prepared by the disclosed method is not ideal and adversely affects the selectivity for heavy oil and coke.
Peoples Republic of China Document CN1429883 discloses a preparation method of in-situ crystallized product with high content of molecular sieves possessing strong resistance to heavy metals. The specific surface areas of the disclosed examples show that the method fails to achieve the objective of increasing molecular sieve content in the crystallized products.